1. Field of the Invention
The present invention relates to an exhaust system of a motorcycle including an exhaust control system for improving an output performance of an engine thereof at an intermediate or low engine revolution range.
2. Related Art
There has been known a four-stroke-cycle multi-cylinder engine having an exhaust system in which exhaust pipes extending from cylinders are collected (or assembled) into a common exhaust pipe. This arrangement permits a negative pressure to be generated in an exhaust pipe of a cylinder which is in its suction phase due to an enhanced pulsation effect of exhaust gases, thereby achieving higher inertia charging efficiency. The pulsation effect generally varies depending on the engine speed (i.e., engine revolution range), and therefore, there has also been proposed an engine having an exhaust control system for controlling an exhaust pulsation by means of, for example, a valve disposed in an exhaust system so as to vary the cross-sectional area of an exhaust passage.
Motorcycles having exhaust control systems have also been proposed. For instance, Japanese Patent Laid-open Publication No. SHO 63-16127 (referred to as a xe2x80x9cprior art reference 1xe2x80x9d) and Japanese Patent Laid-open Publication No. HEI 6-280577 (referred to as a xe2x80x9cprior art reference 2xe2x80x9d) disclose motorcycles each of a type having an exhaust control system provided with a valve disposed in an exhaust collecting portion (exhaust pipe collection (assembling) section or portion) connected to the downstream ends of exhaust pipes. Likewise, Japanese Utility Model Laid-open Publication No. HEI 4-17138 (referred to as a xe2x80x9cprior art reference 3xe2x80x9d) shows a motorcycle in which a valve of an exhaust control system is disposed at the downstream end of a muffler. Furthermore, Japanese Patent Laid-open Publication No. SHO 63-75311 (referred to as a xe2x80x9cprior art reference 4xe2x80x9d) discloses an exhaust control system incorporated in the exhaust system of a two-stroke-cycle engine.
Provision of the exhaust control valve in the exhaust collecting portion downstream of the exhaust pipes, as proposed in the above prior art references 1 and 2, effectively enhances the pulsation effect at low or intermediate engine speed range. However, at a high speed of an engine operation, an exhaust gas flow velocity is still very high in the exhaust collecting portion. Therefore, the exhaust control system, which is disposed in a region where the exhaust gases from the cylinders are collected in each other, undesirably increases resistance against flow of the exhaust gases in the exhaust pipes, resulting in a reduction of the engine output power, thus being inconvenient.
Furthermore, in the prior art, since the collecting portion of the exhaust pipes is disposed under the engine, the provision or location of the exhaust control system in the exhaust collecting portion will disadvantageously restrict an entire volume of an oil pan of an engine unit and adversely affect the shape of cowling of a body of the motorcycle, as well as maximum bank angle thereof.
Moreover, provision or location of the exhaust control system at the downstream end of a muffler as proposed in the above prior art reference 3 cannot fully make use of the pulsation effect, because the pressure of the exhaust gas has been significantly lowered before the gas reaches the downstream end of the muffler, which is also disadvantageous.
Still furthermore, a two-stroke-cycle engine to which the above prior art reference 4 pertains has an exhaust system which is basically different from that of a four-stroke-cycle engine. For instance, in the two-stroke-cycle engine, the exhaust pipes are usually not combined into one unit.
It is an object of the present invention to substantially eliminate defects or drawbacks encountered in the prior art references mentioned above and to provide an exhaust system including an improved exhaust control system of a motorcycle, capable of effectively enhancing an engine output at low or intermediate engine speed range by effectively utilizing a pulsation effect of an exhaust gas, while reducing resistance against the flow of the exhaust gas in the exhaust pipes at a high speed operation of the engine.
The above and other objects can be achieved according to the present invention by providing an exhaust system of a motorcycle which has a body frame including a head pipe supporting a front wheel of the motorcycle, a main frame which branches, at a position immediately behind the head pipe, into a pair of lateral main frame members extending obliquely rearward and downward, a center frame including a pair of lateral center frame members connected to the rear ends of the main frame members so as to extend substantially upright, and a pair of lateral seat rails extending obliquely upward and rearward from end portions of the center frame members, and in which a swing arm supporting a rear wheel of the motorcycle is supported swingably and resiliently to the body frame by means of rear shock absorber, and a four-stroke-cycle multi-cylinder engine is mounted on to the body frame,
the exhaust system comprising:
exhaust pipes leading from the respective cylinders of the engine, the exhaust pipes being collected as a single exhaust pipe collection unit;
a muffler to which the exhaust pipe collection unit is operatively connected;
an interconnecting pipe unit through which the exhaust pipes and the muffler are connected; and
an exhaust control system for controlling pulsation of an exhaust gas, said exhaust control system including a valve mechanism disposed in operative association with the interconnecting pipe unit so as to vary a cross-sectional area of an exhaust passage, and a drive unit for operating the valve mechanism.
A central processing unit for controlling the drive unit may be further provided for the exhaust control system.
In a preferred embodiment of the above aspect, the valve mechanism is located at a position under the swing arm behind the main frame within a triangular zone formed between the rear shock absorber and a bank line that indicates a maximum bank angle of a body of the motorcycle. The valve mechanism may be located at a position on a rear side of the exhaust pipe collection unit and above a road clearance level of the motorcycle body.
The valve mechanism includes a valve member for varying the cross-sectional area of the exhaust passage and a valve shaft carrying the valve member, the valve shaft having an axis extending substantially perpendicularly to the bank line.
The four-stroke-cycle multi-cylinder engine is a four-stroke-cycle four cylinder engine, the exhaust pipes extending from the four cylinders include two pipes disposed to each lateral side of the motorcycle body, which are collected into one exhaust pipe section on each lateral side, the exhaust pipe sections on both lateral sides are collected (assembled) into the single exhaust pipe collection unit connected to the interconnecting pipe.
The motorcycle is provided with a cowling which covers at least a portion of the body frame and which has a rear lower end portion shaped to cover at least a front lower end portion of the valve mechanism.
According to the structure of the present invention mentioned above, the resistance in the exhaust pipes at the engine high revolution operation (high speed operation) can be significantly reduced, and moreover, the output thereof can be improved at the engine intermediate or low revolution operation range by the utilization of the pulsation effect.
According to the arrangement of the valve mechanism of the present invention, the dead space can be effectively utilized in comparison with the prior art arrangement without adversely affecting peripheral equipments or like.
Furthermore, the valve mechanism is arranged above the lowermost position of the motorcycle body behind the exhaust pipe collection unit, so that any adverse affect will not be imparted to the engine lower portion, cawling shape, bank angle and so on of the motorcycle.
Furthermore, according to the present invention, the drive cable can be easily located and operated.
The nature and further characteristic features of the present invention will be made more clear from the following descriptions made with reference to the accompanying drawings.